In laser printers, a desired image is created by causing a laser to be modulated in accordance with a bit pattern stored in an image bit map. The modulated laser beam is scanned across a charged surface of a photosensitive drum in a succession of raster scan lines. Each scan line is divided into pixel areas and the modulated laser beam causes some pixel areas to be exposed to a light pulse and some not, thus causing a pattern of overlapping pixels on each scan line. Where a pixel area is illuminated, the photosensitive drum is discharged, so that when it is subsequently toned, the toner adheres to the discharged areas and is repelled by the still charged areas. Subsequently, the toner is transferred to paper and fixed in the known manner. As a result of the images's pixel configuration, image edges that are either not parallel to the raster scan direction or not perpendicular to it appear stepped.
The prior art has attempted, in a variety of ways, to overcome the stepped appearance of pixel image edges. One of the more widely used techniques is described in U.S. Pat. No. 4,847,641 to Tung, assigned to the Assignee of this application, the disclosure of which is incorporated herein by reference.
Tung shows a character generator that produces a bit map of image data and inputs that bit map into a first-in first-out (FIFO) data buffer. A fixed subset of the buffer stored bits forms a sampling window through which a selected block of the bit map image data may be viewed (for example, a 9.times.9 block of pixels with the edge pixels truncated). The sampling window contains a center bit cell which changes on each shift of the image bits through the FIFO buffer. As the serialized data is shifted, the sampling window views successive bit patterns formed by pixels located at the window's center bit cell and its surrounding neighbor bit cells. Each bit pattern formed by the center bit and its neighboring bits is compared in a matching network with prestored templates. If a match occurs--indicating that the center bit resides at an image edge and that the pixel it represents can be altered so as to improve the image's resolution, a modulation signal is generated that causes the laser beam to alter the center pixel configuration. In general, the center pixel is made smaller than a standard unmodified bit map pixel and is possibly moved within the confines of pixel cell. The pixel size alteration is carried out by modulating the laser contained in a "laser print engine" of the printer. The system taught by Tung is now generally referred to as Resolution Enhancement Technology (RET) and enables substantially improved image resolutions to be achieved.
In laser printers that produce black/white images, the circuitry that modulates the laser beam is directly accessible to the RET circuitry. As a result, variable length pulses generated by an RET circuit can be directly applied to modulate pixel size and to achieve the enhanced image resolution. However, in color laser printers, such direct access is, in general, no longer available. In other words, the laser's drive circuits are not available for external connection to the RET's variable width pulse generator so as to enable modification of pixel size and positioning.
In a color printer, i.e. one which prints successive yellow, magenta, cyan and black "planes", a processor is employed as the laser engine controller. The processor enables the necessary print engine control functions to be carried out in correct time order, while assuring accurate registration of succeeding color planes. To the inventor's knowledge, Resolution Enhancement Technology, as taught by Tung, has not been heretofore accomplished with a color printer.
Accordingly, it is an object of this invention to provide for the incorporation of Resolution Enhancement Technology into a color laser printer.
It is a further object of this invention to incorporate Resolution Enhancement Technology into a laser printer without requiring direct access to laser drive circuits.
It is yet another object of this invention to implement Resolution Enhancement Technology in a color laser printer, without requiring additional circuitry.